feat: Move alembic config into mealie package for easier distribution

[chishm]

What type of PR is this?

• cleanup
• feature

What this PR does / why we need it:

The Alembic configuration and database migration scripts are moved inside the mealie Python package. This means they will be always available, in a consistent location. The advantages are:

• No extra step needed during mealie installation to add the Alembic configuration and scripts.
• No need to guess where the configuration is, so the ALEMBIC_CONFIG_FILE environment variable doesn't need to be set, ever. I've left it in as an option for people who still want to use it.

As a side effect, ruff (specifically isort) no longer thinks alembic is a project ("LOCALFOLDER") package, so the imports of alembic are moved to the third-party group.

Which issue(s) this PR fixes:

This is one task from discussion #4322.

Testing

I built and ran the Docker container, then made sure the database migration still occurred.

I created a database change following the instructions in database-changes.md.

[github-actions]

This PR is stale because it has been open 45 days with no activity.

[chishm]

I'm waiting on #4551 to be reviewed and merged before I rebase this PR for merge.

But if someone wants to review this one first, I'm happy to rebase now.